The Descent of Man

through sexual selection in relation to the propagation
of the species, as well as through natural
selection in relation to the general purposes
of life. Hence secondary sexual characters,
when equally transmitted to both sexes can be
distinguished from ordinary specific characters
only by the light of analogy. The modifications
acquired through sexual selection are often so
strongly pronounced that the two sexes have
frequently been ranked as distinct species, or
even as distinct genera. Such strongly-marked
differences must be in some manner highly
important; and we know that they have been
acquired in some instances at the cost not only
of inconvenience, but of exposure to actual
danger.
The belief in the power of sexual selection rests
chiefly on the following considerations. Certain
characters are confined to one sex; and this alone
renders it probable that in most cases they
are connected with the act of reproduction. In